Q: How do dunes form and grow?

They form over flat terrain and grow by the sand first accumulating around some obstruction and then growing with a windward face ( or slight slope ) and a slipface ( with a steeper slope ).

Did you know that there are also snow dunes?

They form in the same way as sand dunes and their shapes are also referred to as Barchan, longitudinal and stars where the winds affect them to be so. Greenland has many shaped snow dunes, especially along the coastal areas.

DesertUSA also has some great dune information.